The present application is a U.S. national phase of PCT/GB97/00660, filed Mar. 11, 1997, which claims the benefit of GB 9605222.0, filed Mar. 12, 1996.
The present invention relates to novel peptides capable of eliciting an immunological response that is protective against Clostridium perfringens epsilon toxin in man or animals. It relates to the production of these peptides and to pharmaceutical compositions containing them, Preferred agents enable prophylaxis and treatment of Clostridium perfringens induced disease states in both humans and other animals.
Clostridium perfringens (C. perfringens) is ubiquitous in the environment and has been found in the soil, decaying organic matter and as part of the gut flora in man and animals. Different strains of C. perfringens can be assigned to one of five biotypes (A-E) depending on the spectrum of types produced see McDonel, J. L. (1986); Toxins of Clostridium perfringens types A,B,C,D and E. In Pharmacology of Bacterial Toxins; F. Dorner and J. Drews, eds. (Oxford: Pergamon Press), pp. 477-517. The epsilon toxin is produced by C. perfringens types B and D but not by types A, C or E see Brooks, M. E., Sterne, M., and Warrack, G. H. (1957); A reassessment of the criteria used for type differentiation of Clostridia perfringens. J. Pathol. Bacteriol. 74, 185-195. C. perfringens types B and D have a limited host range being mainly isolated from goats and cattle and rarely from man, Smith, L. D. and Williams, B. L. (1984); The pathogenic anaerobic bacteria (Springfield, Ill.: Charles C. Thomas). They are responsible for producing severe and rapidly fatal enterotoxaemia: C. perfringens type B enterotoxaemia infection of lambs causes lamb dysentery while type D enterotoxaemia produces pulpy kidney disease in sheep and lambs. Mortality rates in both cases may be as high as 100%. Neither disease is infectious, but sporadic outbreaks occur when the microbial balance of the gut is disrupted, for example after antibiotic treatment or due to changes in diet. Pulpy kidney disease is often associated with a change from a poor to a rich diet accompanied by excessive over-eating, Bullen, J. J. (1970); Role of toxins in host-parasite relationships. In Micribial toxins volume 1. S. Ajl, S. Kadis, and T. C. Montie, eds. (New York: Academic Press), pp. 233-276. Such over-eating causes considerable quantities of undigested, starch-rich food to pass from the rumen into the small intestine. The nutritious anaerobic environment this produces allows the multiplication of C. perfringens resulting in up to 109 cfu per g of ileal contents and high concentrations of epsilon toxin Bullen, J. J. and Scarisbrick, R. (1957); Enterotoxaemia of sheep: experimental reproduction of the disease; J. Pathol. Bacteriol. 73, 494-509. Several vaccines exist for the prevention of C. perfringens enterotoxaemia. The vaccines are based on formaldehyde-treated cell filtrates or whole cell cultures. The vaccines confer a high degree of protection in animals Stephen, J. and Pietrowski, R. A. (1986); Bacterial toxins (England: van Nostrand Reinhold (UK) Co. Ltd.); however, the immunogenicity of the epsilon toxin in the preparations has been reported to be variable and a more defined and consistent vaccine is preferable. Immunity to a single epitope on the toxin has been shown to be sufficient to protect against purified epsilon toxin and C. perfringens infection, Percival, D. A., Shuttleworth, A. D., Williamson, E. D., and Kelly, D. C. (1990), Anti-idiotypic antibody-induced protection against Clostridium perfringens type D; Infect. Immun. 58, 2487-2492.
Epsilon toxin is produced by C. perfringens types B and D as a relatively inactive prototoxin of 311 amino acids with a molecular weight of 32,700, Worthington, R. W. and Mulders, M. S. (1977); Physical changes in the epsilon prototoxin molecule of Clostridium perfringens during enzymatic activation; Infect. Immun. 18, 549-551. Proteolytic cleavage of 13 or 14 basic amino acids from the amino terminal of the prototoxin results in the production of the mature toxin with a molecular weight of 31,200 Worthington and Mulders, 1977; Hunter, S. E., Clarke, I. N., Kelly, D. C., and Titball, R. W. (1992); Cloning and nucleotide sequencing of the Clostridium perfringens epsilon-toxin gene and its expression in Escherichia coli; Infect. Immun. 60, 102-110. Activation also results in a marked shift in pI from 8.02 (prototoxin) to either 5.36 (fully active toxin) or 5.74 (partially active toxin) and a significant change in conformation (Worthington and Mulders, 1977; Habeeb, A. F., Lee, C. L., and Atassi, M. Z. (1973); Conformational studies on modified proteins and peptides, VII; Conformation of epsilon-prototoxin and epsilon-toxin from Clostridium perfringens; Conformational changes associated with toxicity; Biochim. Biophys. Acta 322, 245-250). A complication is that the activation of the prototoxin seems to produce several isoforms with a range of specific activities between that of the prototoxin and the mature toxin (Habeeb, A. F. (1975); Studies on epsilon-prototoxin of Clostridium perfringens type D. Physicochemical and chemical properties of epsilon-prototoxin; Biochim. Biophys. Acta 412, 62-69; Worthington and Mulders, 1977). More recently it has been found that the toxin itself also has several isoforms (Hunter et al., 1992). Thus activation of epsilon prototoxin may be a multi-step process, possibly with multiple proteolytic cleavages and post-translational modifications such as deamination and phosphorylation resulting in the production of the heterogeneous mature toxin (Hunter et al., 1992).
Epsilon toxin is usually obtained from a type D strain of C. perfringens and has been purified either individually or in combination by methanol precipitation, ammonium sulphate precipitation, column chromatography, size exclusion and various forms of ion exchange chromatography (Verwoerd, D. W. (1960); Isolation van die protoksien van Clostidium welchii type D. J. S. Afr. Vet. Med. Assoc. 31, 195-203; Habeeb, A. F. (1969); Studies on epsilon-prototoxin of Clostridium perfringens type D. I. Purification methods: evidence for multiple forms of epsilon-prototoxin; Arch. Biochem. Biophys. 130, 430-440; Worthington, R. W., Mulders, M. S., and Van Rensburg, J. J. (1973); Clostridium perfringens type D epsilon prototoxin. Some chemical, immunological and biological properties of a highly purified prototoxin; Onderstepoort. J. Vet. Res. 40, 143-149; Payne, D. W., Williamson, E. D., Havard, H., Modi, N., and Brown, J. (1994); Evaluation of a new cytotoxicity assay for Clostridium perfringens type D epsilon toxin; FEMS Microbiol. Lett. 116, 161-167).
Traditionally, the activity of purified epsilon toxin has been determined in mouse lethality tests (Habeeb, A. F. (1969); Studies on epsilon-prototoxin of Clostridium perfringens type D. I. Purification methods: evidence for multiple forms of epsilon-prototoxin; Arch. Biochem. Biophys. 130, 430-440; Worthington, R. W., Mulders, M. S., and Van Rensburg, J. J. (1973); Clostridium perfringens type D epsilon prototoxin. Some chemical, immunological and biological properties of a highly purified prototoxin; Onderstepoort. J. Vet. Res. 40, 143-149). The mature toxin is highly toxic with an LD50 in mice of  less than 100 ng when administered intravenously (Payne, D. W., Williamson, E. D., Havard, H., Modi, N., and Brown, J. (1994); Evaluation of a new cytotoxicity assay for Clostridium perfringens type D epsilon toxin; FEMS Microbiol. Lett. 116, 161-167). As the basis of an alternative assay for epsilon toxin activity, it has been found that the Madin Darby Canine Kidney (MDCK) cell line was sensitive to C. perfringens type D culture filtrates (Knight, P. A., Burnett, C., Whitaker, A. M., and Queminet, J. (1986); The titration of clostridial toxoids and antisera in cell culture; Develop. biol. Standard. 64, 129-136). It was demonstrated that the lethal and dermonecrotic effects of the toxin observed in rabbits and its cytopathic activity were all caused by the same entity in epsilon toxin preparations and that all three activities were valid indicators in toxin neutralisation tests (Knight, P. A., Queminet, J., Blanchard, J. H., and Tilleray, J. H. (1990); In vitro tests for the measurement of clostridial toxins, toxoids and antisera. II. Titration of Clostridium perfringens toxins and antitoxins in cell culture; Biologicals. 18, 263-270). Recently, the development of a new cytotoxicity assay for the determination of the activity of C. perfringens type D epsilon toxin based on the sensitivity of the MDCK cell line has been reported (Payne, D. W., Williamson, E. D., Havard, H., Modi, N., and Brown, J. (1994); Evaluation of a new cytotoxicity assay for Clostridium perfringens type D epsilon toxin; FEMS Microbiol. Lett. 116, 161-167). In four out of five samples between 15-22 ng/ml of purified epsilon toxin was sufficient to reduce the viability of MDCK cells by 50% and as little as 8 ng/ml sufficient to cause a significant reduction in the viability of the MDCK cells, Payne et al., 1994.
The etx gene encoding epsilon toxin is carried out on an episome distinct from the 3.6Mb chromosome (Canard, B., Saint Joanis, B., and Cole, S. T. (1992); Genomic diversity and organization of virulence genes in the pathogenic anaerobe Clostridium perfringens. Mol. Microbiol. 6, 1421-1429). The gene has been cloned and sequences for both B and D types determined. The cloned gene etxB coded for a protein of Mrxcx9c32,981 (Hunter, S. E., Clarke, I. N., Kelly, D. C., and Titball, R. W. (1992); Cloning and nucleotide sequencing of the Clostridium perfringens epsilon-toxin gene and its expression in Escherichia coli; Infect. Immun. 60, 102-110). Neither the sequenced gene or the derived protein showed homology with other proteins. Comparison of the sequences of cloned etx genes from type B and type D strains revealed two nucleotide differences in the open reading frame resulting in one amino acid substitution (Havard, H. L., Hunter, S. E., and Titball, R. W. (1992); Comparison of the nucleotide sequence and development of a PCR test for the epsilon toxin gene of Clostridium perfringens type B and type D; FEMS Microbiol. Lett. 76, 77-81). The promoters for the genes were not homologous, with different putative xe2x88x9210 and xe2x88x9235 sequences. This allowed the development of epsilon-specific PCR primers to produce a system for typing B and D strains of C. perfringens. The etx promoter allowed expression of the cloned gene in E. coli (Hunter et al., 1992). Epsilon toxin is preceded by a signal peptide resulting in the native protein being exported from C. perfringens and the recombinant protein accumulating in the periplasmic space of E. coli (Hunter et al., 1992; Bullen, J. J. and Batty, I. (1956); The effect of Clostridium welchii type D culture filtrates on the permeability of the mouse intestine; J. Pathol. Bacteriol. 71, 311-323). The recombinant toxin expressed in E. coli was shown to have identical biochemical and biological properties to those of the native toxin.
Epsilon prototoxin produced in the gut of animals is activated by proteolytic enzymes present in intestinal fluid (Niilo, L. (1965); Bovine enterotoxaemia.III; Factors affecting the stability of the toxins of Clostridium perfringens types A, C and D; Can. Vet. J. 6, 38-42). The mature toxin increases intestinal permeability and enters the blood supply (Bullen and Batty, 1956; Bullen, J. J. (1970); Role of toxins in host-parasite relationships. In Micribial toxins volume 1. S. Ajl, S. Kadis, and T. C. Montie, eds. (New York: Academic Press), pp. 233-276; Jansen, B. C. (1967); The production of a basic immunity against pulpy kidney disease; Onderstepoort. J. Vet. Res. 34, 65-80. The mode of action of epsilon toxin is not known, but several observations have suggested that it acts upon the central nervous system. The toxin rapidly causes a widespread disturbance in the permeability balance of the brain by disrupting vascular endothelia (Finnie, J. W. (1984); Ultrastructural changes in the brain of mice given Clostridium perfringens type D epsilon toxin; J. Comp. Pathol. 94, 445-452; Buxton, D. (1976); Use of horseradish peroxidase to study the antagonism of Clostridium welchii (Cl. perfringens) type D epsilon toxin in mice by the formalinized epsilon prototoxin; J. Comp. Pathol. 86, 67-72). As degenerative changes progress, serum proteins and eventually red cells leak from the vasculature, astrocyte end feet rupture and oedema ensues (Buxton, D. and Morgan, K. T. (1967); Studies of the lesions produced in the brains of colostrum deprived lambs by Clostidium welchii (Clostridium perfringens) type D toxin; J. Comp. Path. 86, 435-447). In acute cases of epsilon toxin induced entertoxaemia characteristic lesions occur at specific sites in the brain (Hartley, 1956; Buxton, 1976; McDonel, 1986). Chemical modification experiments have demonstrated the importance of certain amino acid residues for the lethality of epsilon toxin. One tryptophan (Sakurai, J. and Nagahama, M. (1985); Role of one tryptophan residue in the lethal activity of Clostridium perfringens epsilon toxin; Biochem. Biophys. Res. Commun. 128, 760-766), one histidine (Sakurai, J. and Nagaharna, M. (1987); Carboxyl groups in Clostridium perfringens epsilon toxin; Microb. Pathog. 3, 469-474), one tyrosine (Sakurai, J. and Nagahama, M. (1987); The inactivation of Clostridium perfringens epsilon toxin by treatment with tetranitromethane and N-acetylimidazole; Toxicon 25, 279-284) and three or four aspartic or glutamic acids (Sakurai, J. and Nagahama, M. (1987); Histidine residues in Clostridium perfringens epsilon toxin; FEMS Microbiology Letters 41, 317-319) residues were shown to be essential for the lethal effect of epsilon toxin. Eight lysine residues have also been shown to be important in activity, but are probably involved in maintaining conformation rather than being integral to an active site (Sakurai, J. and Nagahama, M. (1986); Amino groups in Clostridium perfringens epsilon prototoxin and epsilon toxin. Microb. Pathog. 1, 417-423).
It is an object of the present invention to provide novel polypeptides for use in vaccines which are capable of inducing protective antibodies directed against C. perfringens epsilon toxin when administered to animals or man and thereby providing prophylaxis or therapy against infection by C. perfringens epsilon toxin.
The present invention provides a polypeptide capable of producing an immune response which is protective against Clostridium perfringens, said polypeptide comprising an amino acid sequence which has at least 60% homology with the amino acid sequence of Clostridium perfringens epsilon toxin or an immunogenic fragment thereof, characterised in that the amino acid residue corresponding to residue 106 of the mature toxin is of an amino acid other than histidine.
Suitably, the polypeptide has an amino acid sequence which has at least 80% homology and preferably 90% homology and is most preferably substantially completely homologous with the amino acid sequence of Clostridium perfringens epsilon toxin or an immunogenic fragment thereof.
The amino acid sequence of Clostridium perfringens epsilon toxin is shown hereinafter in as amino acids 1-283 of FIG. 2 (SEQ ID No 2). Where the polypeptide of the invention is homologous to that of SEQ ID No 2 or an immunogenic fragment thereof, it is preferable that any altered amino acids are replaced by conservative substitutions.
By xe2x80x98conservative substitutionxe2x80x99 is meant the substitution of an amino acid by another one of the same class; the classes being as follows:
As is well known to those skilled in the art, altering the primary structure of a peptide by a conservative substitution may not significantly alter the a ctivity of that peptide because the side-chain of the amino acid which is inserted into the sequence may be able to form sim ilar bonds and contacts as the side chain of the amino acid which has been substituted out. This is so even when the substitution is in a region which is critical in determining the peptides conformation.
Non-conservative substitutions are possible provided that these do not interupt with the immunogenicity of the polypeptide.
The expression xe2x80x9cimmunogenic fragmentxe2x80x9d used herein refers to a polypeptide which is shorter than full length native toxin, but which includes at least one antigenic determinant and also which includes a residue corresponding to residue 106 of the mature toxin. Suitably the fragments will comprise at least 15, more suitably at least 30 and preferably a t least 60 amino acids.
In particular, the polypeptide comprises a protein which has an amino acid sequence which has at least 60% homology with the amino acid sequence of Clostridium perfringens epsilon toxin characterised in that the amino acid residue corresponding to residue 106 of the mature toxin is of an amino acid other than histidine.
Most preferably the protein comprises the amino acid sequence of clostridium perfringens epsilon toxin and is characterised in that the amino acid residue corresponding to residue 106 of the mature toxin is of an amino acid other than histidine.
Preferably the amino acid at position 106 is a non-basic amino acid, and in particular a non polar amino acid, especially proline.
The polypeptides or proteins of the invention are genetically toxoided (inactivated) which means that they are less likely to cause unwanted side effects in animals to which they are administered. This is a much more precise and quantifiable way of inactivating the toxin rather than using chemical toxoiding methods.
It should also be stressed that the invention also encompasses peptides comprising the amino acid sequences described above i.e. wherein the N- or C- terminus has been extended. Extension of the peptides above may confer additional desirable properties on them, for instance, easier separation or purification, or enhancing or adding to the immunity or labelling.
In particular, the polypeptide or protein described above may form part of a fusion protein which may further comprise a moiety which confers these additional properties. For example, the amino acid sequence of glutathione-S-transferase may be included or A non-C. perfringens antigenic protein may be included fused to the protein of the invention for the purpose of providing other immunity or labelling. Alternatively the polypeptide or protein of the invention may be in the form of a conjugate with another protein which confers such an additional desirable property.
The polypeptides of the invention may be prepared synthetically, or more suitably, they are obtained using recombinant DNA technology. Thus the invention further provides a nucleic acid which encodes a polypeptide as described above.
Suitably, the nucleic acid comprises the part of the sequence shown in SEQ ID No 5 which encodes the SEQ ID no 6.
Such nucleic acids may be incorporated into an expression vector, such as a plasmid, under the control of a promoter as understood in the art. The vector may include other structures as conventional in the art, such as signal sequences, leader sequences and enhancers, and can be used to transform a host cell, for example a prokaryotic cell such as E. coli or a eukaryotic cell. Transformed cells can then be cultured and polypeptide of the invention recovered therefrom, either from the cells or from the culture medium, depending upon whether the desired product is secreted from the cell or not.
In a further aspect of the invention there is provided a method for inducing an immune response protective against Clostridium perfringens epsilon toxin in a mammal, said method comprising administering to said mammal an polypeptide as described above.
Suitable mammals include humans and animals, such as sheep, lambs and goats.
The polypeptide may be administered to the mammal directly, for example in the form of a vaccine composition. Alternatively, a nucleic acid encoding it may be incorporated into a suitable vaccine vector, for example an attenuated live virus vaccine carrier under the control of suitable promoters etc. to ensure that the vector expresses the polypeptide in situ. Administration of the vector to the mammal thereby produces the desired immune response. Suitable vectors will be apparent to the skilled person. They may include vaccinia virus vectors, such as the Lister strain, or attenuated gut-colonising microorganisms such as attenuated strains of Salmonella.
In a further embodiment the present invention provides vaccine compositions comprising the polypeptides or proteins of the invention or as an alternative, a vector capable of expressing said polypeptide or protein, suitably in appropriate dosage units. The compositions are optionally complemented as necessary by further agents for optimising protection eg adjuvants and carriers, preferably pharmaceutically acceptable carriers and adjuvants. Freunds incomplete or complete adjuvant or alhydrogel may be used as typical adjuvants, but other suitable candidates such as those described in WO 9203164 may be used. Carrier function may be fulfilled by saline solutions. The carrier may be one suited to parenteral administration, particularly intraperitoneal administration but optionally oral for example in a live vaccine vector such as an attenuated gut-colonising micro-organism, or administration in the form of droplets or capsules, such as liposomes or microcapsules as would be effective in delivering the composition to the airways of an individual for the purpose of evoking a mucosal immune response.
The microcapsule may comprise biodegradable polymers for example polylactic acid either with or without glycolic acid or with or without a block co-polymer which may contain the following repeat unit: (POP-POE)n where POP is polyoxypropylene and POE is polyoxyethylene. Block co-polymers which contain (POP-POE)n may be of particular use.
The proteins and fusion proteins of the present invention may be used as mucosal adjuvants. They may be co-administered with a non-C. perfringens antigenic proteinxe2x80x94this may augment the mucosal immune response to the non-C. perfringens antigenic protein. There is evidence that epsilon toxin binds to a cell surface receptor in Payne et al 1994.